Aircraft: B737
Phase of flight: Parked
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy

A B737-700 aircraft; Aircraft X; was in ZZZ for a Heavy C-1 (HC1) Check. I; Inspector X; was assigned to Heavy Tail Inspection. I found two each cracks on the Left-Hand (L/H) Horizontal Stabilizer lower inspar skin. Mechanics started the repair per Structural Repair Manual (SRM) XX-XX-01-2R-11; Repair 11. Per the SRM repair; Table 201 calls out the Repair Doubler to use 2024-T3 and Table 202 calls out for the Doubler to be .090' thousandths of an inch thick. As the night went on I stopped by to verify metal type and thickness and saw the X-colored Tag (Form) showing [2023]-T3; .090' thick on the 16'x16' inch piece on metal the mechanics had. I told them to carry on with the repair and parts fabrication (fab). During the fab of the repair parts; it was time to turn over [the work] to dayshift. At that time; I and the mechanics signed-off the steps on the (Form). The graveyard shift ended and the mechanics went home. I stayed and worked the aircraft. Dayshift mechanics took over the repair part fabrication and continued to install. At one point during the dayshift; the mechanics working the repair asked if the Doubler was in fact [2024]-T3; I responded; 'Yes.' I saw the X-colored tag (form) during the night shift. Mechanic X stated that it didn't appear correct; but with the information I told him; he continued on with the repair. With all solid fasteners installed and waiting on an Engineering Authorization (E/A) from Engineering for two blind fasteners to be installed; I then turned the aircraft over. [Several shifts later]; the dayshift Mechanic X that installed the repair on L/H Horiz Stab lower inspar skin on Aircraft X; needed some metal from the Sheet Metal (S/M) Shop. He then noticed that a sheet of [2024]-0 was mislabeled as [2024]-T3. He brought this to my attention and we realized that this was the same metal used on Aircraft X. At this time; we called the company Hot Line and the aircraft was grounded. [Recommend] better Vendor Quality Control (Q/C). I; Mechanic X; took a [Maintenance] Turnover from the grave shift B/C Check [Crew] on Aircraft X; a B737-700 aircraft for a Left-Hand (L/H) Horizontal Stab lower skin crack. The mechanics that worked it [on grave shift] had the Repair Doubler fabricated; all I had to do was countersink the fastener holes and install the repair. After installation; I noticed that the Repair Doubler had dimpled when I shot it on. I found this to be odd. I brought this to the attention of the Inspector X; he assured me that he had checked the [X-colored] Tag on the metal and it was [2024]-T3. [Several shifts later] while working on another aircraft; I noticed a piece of [2024]-O material in the Sheet Metal (S/M) Shop that was mislabeled [2024]-T3. At that time I realized that the wrong metal was installed on Aircraft X on the L/H Horizontal Stab lower skin. I then informed Inspector X that I had worked with on Aircraft X; and we contacted Maintenance Control and had the aircraft taken out of service. Upon further investigation; we found the Certifications (Certs) from the metal Vendor were wrong. The Certs said the metal was [2024]-T3; when in fact it was [2024]-O. Dayshift. [Recommend] while selecting metal for a repair; verify Tag and stencil on metal match. An Aircraft Maintenance Inspector; a Parts Receiving Inspector; and an Aircraft Maintenance Technician (AMT) report about a Structural Repair Doubler that was inadvertently fabricated and installed from a mislabeled sheet of 2024-0 aluminum instead of the harder 2024-T3 ALCLAD aluminum material. Doublers are being installed over the Horizontal Stab skin cracks found on many B737-700 aircraft.